<HTML>
<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A> 
</CENTER>






<HR>

<H3>dihedral_style harmonic command 
</H3>
<H3>dihedral_style harmonic/omp command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>dihedral_style harmonic 
</PRE>
<P><B>Examples:</B>
</P>
<PRE>dihedral_style harmonic
dihedral_coeff 1 80.0 1 2 
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>harmonic</I> dihedral style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/dihedral_harmonic.jpg">
</CENTER>
<P>The following coefficients must be defined for each dihedral type via the
<A HREF = "dihedral_coeff.html">dihedral_coeff</A> command as in the example above, or in
the data file or restart files read by the <A HREF = "read_data.html">read_data</A>
or <A HREF = "read_restart.html">read_restart</A> commands:
</P>
<UL><LI>K (energy)
<LI>d (+1 or -1)
<LI>n (integer >= 0) 
</UL>
<P>IMPORTANT NOTE: Here are important points to take note of when
defining LAMMPS dihedral coefficients for the harmonic style, so that
they are compatible with how harmonic dihedrals are defined by other
force fields:
</P>
<UL><LI>The LAMMPS convention is that the trans position = 180 degrees, while
in some force fields trans = 0 degrees. 

<LI>Some force fields reverse the sign convention on <I>d</I>. 

<LI>Some force fields let <I>n</I> be positive or negative which corresponds to
<I>d</I> = 1 or -1 for the harmonic style. 
</UL>
<HR>

<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
the same as the corresponding style without the suffix.  They have
been optimized to run faster, depending on your available hardware, as
discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
manual.  The accelerated styles take the same arguments and should
produce the same results, except for round-off and precision issues.
</P>
<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
packages, respectively.  They are only enabled if LAMMPS was built with
those packages.  See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
section for more info.
</P>
<P>You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
switch</A> when you invoke LAMMPS, or you can
use the <A HREF = "suffix.html">suffix</A> command in your input script.
</P>
<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
more instructions on how to use the accelerated styles effectively.
</P>
<HR>

<P><B>Restrictions:</B>
</P>
<P>This dihedral style can only be used if LAMMPS was built with the
MOLECULAR package (which it is by default).  See the <A HREF = "Section_start.html#start_3">Making
LAMMPS</A> section for more info on packages.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "dihedral_coeff.html">dihedral_coeff</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
